1. Field of the Invention
The present invention is related to an image forming apparatus such as a copying machine, a printing machine and the like which provides a plurality of developing devices therein.
2. Description of the Prior Art
There has been known an image forming apparatus with a plurality of developing devices. In an image forming apparatus of this type, a plurality of developing devices are arranged circumferentially around an image forming drum which are selectively used to obtain a copy of a desired color such as black, red and/or green etc.
In such an image forming apparatus, the developing device containing black toner is used frequently since black color is ordinarily designated by the user. Therefore, the fluidity of developer contained therein is maintained in a good state.
Contrary to the above, since the developing device containing color toner other than black is driven not so often as compared with the developing device for black toner, developer contained therein is apt to be coagulated by the ambient humidity penetrating thereinto.
Once developer was coagulated in the developing device, developer is not supplied smoothly onto the photosensitive layer on which an electrostatic latent image is formed. This causes the density of the image developed to decrease and the image formed becomes blotchy.
Conventionally, it has been proposed to provide a manual switch for operating the developing motor provided for driving the developing device. When the manual switch was operated by the user after the exchange of the toner replenishing container, developer contained therein is stirred and agitated keeping the developing device itself in a non-operational mode in which for instance, only the developing motor is driven while the developing bias is turned off.
However, in that case mentioned above, key or switch operation becomes more and more confusing and, since said manual switch is operated only arbitrarily, there exists a high possibility that developer in the developing device not used so often is left unstirred for a long time to cause a coagulation thereof.
In such an image forming apparatus with a plurality of developing devices, there are provided a plurality of selecting keys for selecting or designating one of them desired and toner sensors each of which is provided for detecting when toner in the corresponding toner replenishing container was exhausted.
Consider a case where one of the toner containers is exchanged for a new one. In such a case, if another developing device is selected other than the developing device in which the toner container was just exchanged, the operator has to operate one of the selecting keys again although he wants to copy with toner contained in the toner container just exchanged.
Moreover, if he forgot the operation, a copy of the color which he did not expect is obtained undesirably.